Compression ignition diesel engines have great utility and advantage as vehicle power trains because of their inherent fuel economy and high torque at low speed. Diesel engines run at a high air to fuel (A/F) ratio under very fuel lean conditions. Because of this, they have very low emissions of gas phase hydrocarbons and carbon monoxide. However, diesel exhaust is characterized by relatively high emissions of nitrogen oxides (NOx) and particulates. The particulate emissions, which are measured as condensed matter at 52° C., are multi phase being comprised of solid (insoluble) carbon soot particles, liquid hydrocarbons in the form of lube oil and unburned fuel, the so called soluble organic fraction (SOF), and the so called “sulfate” in the form of SO3+H2O=H2SO4.
Like in gasoline engine systems, pollution control devices are used in diesel engine systems to reduce pollutants. These devices, which include diesel oxidation catalysts (DOC) and particulate matter (PM) filters, for example, catalyzed soot filters (CSF), are installed in the exhaust system of the engine. As the catalyst which is responsible of hydrocarbon conversion ages, its ability to reduce combustion by-products, referred to as conversion efficiency, diminishes. These combustion by-products include, but are not limited to, carbon monoxide and hydrocarbons.
Various government agencies are requiring vehicles to be equipped with emission monitoring systems, which are commonly referred to as On-Board Diagnostic (OBD) systems, to notify the operator of the vehicle when the emissions exceed government allowed standards. It is required that a malfunction indicator light be turned on when levels of specific emissions exceed the standard by a designated factor. Current OBD requirements are targeted primarily at hydrocarbon (HC) and nitrogen oxides (NOx) emissions.
Diesel vehicles in the United States must fulfill the United States Environmental Protection Agency EPA OBD II and California Air Resources Board (CARB) OBD II requirements meaning that hydrocarbon limits may not be exceeded according to their respective standards as measured during a Federal Test Procedure (FTP) test. In order to fulfill this requirement, the HC conversion needs to be monitored during vehicle operation. Since diesel emissions contain heavier, more condensable HC components than gasoline engine emissions, the heavy HC emissions can adsorb onto the catalyst at temperatures below light-off. Therefore, if HC levels are measured before and after the catalyst for diesel engines, it may appear that HC conversion is taking place when actually the post catalyst HC reduction is due to the HC being stored on the catalyst. Thus, it is not possible to estimate catalyst efficiency in a diesel engine accurately by monitoring HC after the catalyst.
Since there are no measures available to directly detect hydrocarbon emissions during vehicle operation, gasoline powered vehicles utilize an indirect measurement using a correlation between the oxygen storage capacity of the catalyst and its actual conversion performance for hydrocarbons. In gasoline engines that utilize three-way catalysts (TWC), the air to fuel ratio oscillates between fuel rich and fuel lean engine operating conditions. The oxygen storage component (OSC) contained in a three-way-catalyst has the catalytic purpose of maintaining stoichiometric conditions in the catalyst by releasing oxygen into the exhaust under fuel rich engine operating conditions and absorbing oxygen from the exhaust under fuel lean operating conditions. The OSC function thereby enables the optimal oxidation of HCs and CO during fuel rich operating conditions as well as reduction of NOx to N2 under fuel lean operating conditions. The OSC functionality is, therefore, a pre-requisite for the efficient catalytic removal of HCs, CO, and NOx over a TWC under all engine operating conditions. At the same time, OSC ability to release or take up oxygen serves as a means to monitor the catalytic function of the TWC in gasoline OBD systems. This type of monitoring has not been applied so far for diesel engine systems because they are operated under constant fuel lean engine operating conditions making an OSC component unnecessary.
Therefore, current diesel oxidation catalysts are unable to fulfill the current US EPA/CARB OBD requirements because of the lack of ability to measure hydrocarbon conversion during vehicle operation. It would be desirable to provide exhaust treatment systems and methods for diesel engine systems that allowed for such monitoring of the performance of the diesel catalysts.